As a bonding wire for semiconductor used to connect an electrode on a semiconductor device to an external connection terminal (referred to as “bonding wire” hereunder), there has been mainly used, nowadays, a bonding wire (gold bonding wire) having a wire diameter of about 20 to 50 μm and being made of 4N gold (Au) with a high purity (4-Nine, a purity of 99.99% by mass or higher). In general, there has conventionally been performed ball bonding featuring a thermal compressive bonding technique with the aid of ultrasound, in order to bond the gold bonding wire to an electrode on a silicon chip serving as a semiconductor device, i.e., a method in which a general bonding device is used so as to pass the gold bonding wire through a jig called capillary, followed by heating and melting a front end of this bonding wire through an arc heat input, thus forming a ball thereon due to a surface tension, such ball being further pressed and bonded to the aforementioned electrode heated up to a temperature in a range of 150 to 300° C.
In contrast, there has conventionally been performed so-called wedge bonding to bond the gold bonding wire directly to an electrode without forming the aforementioned ball, when connecting the corresponding bonding wire to an external connection terminal such as a lead, a land or the like. In recent years, there have been observed rapid diversifications in a semiconductor mounting structure, material and connection technology. For example, in addition to the existing QFP (Quad Flat Packaging) using a lead frame, there have been put to practical use new mounting methods including: BGA (Ball Grid Array) using a substrate, a polyimide tape and the like; CSP (Chip Scale Packaging) and the like, with the external connection terminal being diversified. For this reason, a wedge bonding property has become more crucial than ever.
Further, since there is a growing need for miniaturization of semiconductor devices and performing thin-mounting accordingly, there have been increasingly employed: a low loop bonding technique featuring a low loop height of a bonding wire; a reverse bonding technique for releasing loops upward toward multilayered chips from a substrate side, and the like.
With resource prices escalating in recent days, there has also been a steep rise in the price of gold employed as a raw material of the gold bonding. Here, copper (Cu) has been considered a substitute for gold as a low-cost material for a bonding wire. However, since copper is more susceptible to oxidation than gold, it is difficult to store a simple copper bonding wire for a long period of time, and there cannot be achieved a favorable wedge bonding property with such bonding wire. Further, a ball formed on a front end of such simple copper bonding wire actually has to be formed under a reductive atmosphere so that the ball thus formed is not oxidized. Specifically, the reductive atmosphere around the ball is generally given using a gas prepared by mixing hydrogen (H2) of about 4% by volume into nitrogen (N2). However, it is still difficult to achieve such a favorable ball bonding property as is the case in the gold bonding wire. For those reasons, the copper bonding wire has not yet been widely employed in the LSI industry overall.
Here, in order to provide a solution to the oxidation of the copper bonding wire, there has been proposed a type of copper bonding wire with silver (Ag) coated on a surface of a copper wire. For example, in a patent document 1, although there is not disclosed a specific example of how a copper wire is coated with silver, there are disclosed kinds of both inner metals and surface coating metals for the bonding wire. Specifically, the inner metals are non-pure noble metals including aluminum (Al), copper, iron (Fe), an alloy of iron and nickel (FeNi) and the like. And, the surface coating metals are metals having corrosion resistance to moisture, salt content, alkalis or the like, such metals including gold and silver. Further, in a patent document 2, although there is not disclosed a specific example of how a copper wire is coated with silver, there is disclosed a copper bonding wire with a noble metal such as gold, silver or the like coated on a copper wire. According to the patent document 2, there can be achieved a further improved corrosion resistance property with the copper wire thus coated. A patent document 3 discloses a bonding wire composed of an aluminum (Al) wire or a copper wire, such aluminum wire or copper wire being further plated with a noble metal such as gold, silver or the like. The copper bonding wire according to the patent document 3 can provide a solution to corrosion and to thermal oxidation due to the plating, and allows a reliable bondability to be achieved with respect to a lead frame as is the case in the gold bonding wire. A patent document 4 discloses a copper bonding wire with a noble metal or anticorrosion metal coated on a surface of a high-purity copper extra fine wire, such noble metal, however, being not limited to silver. This composition allows surface oxidation of the corresponding copper bonding wire to be controlled (particularly, the presence of surface oxidation after being left for 10 days in the atmosphere). Further, a diameter of the aforementioned copper extra fine wire is 15 to 80 μm, and a coating layer formed in this case has an average layer thickness of 10 nm to 1 μm (the diameter of the wire is 25 μm, and the average layer thickness of the coating layer is actually 0.1 μm in an example). A patent document 5 discloses a copper bonding wire with silver coated on a surface of a copper fine wire, such silver coating being 0.001 to 0.01 times thicker than a diameter of the copper fine wire. Namely, when the diameter of the copper fine wire is 25 μm, the thickness of silver coated thereon is 0.02 to 0.3 μm. Such a kind of silver coating not only controls oxidation of copper, but also improves a ball formability.
Further, in order to provide a solution to oxidation of the copper bonding wire, there has been proposed a copper bonding wire with a noble metal, particularly, gold (Au) coated on a surface of a copper wire. For example, in the patent document 1, although there is not disclosed a specific example of how a copper wire is coated with gold, there are disclosed the kinds of both the inner metals and the surface coating metals for the bonding wire. Specifically, the inner metals are non-pure noble metals including aluminum (Al), copper, iron (Fe), an alloy of iron and nickel (FeNi) and the like. And, the surface coating metals are metals having corrosion resistance to moisture, salt content, alkalis or the like, such metals including gold and silver. A patent document 7 discloses a bonding wire composed of a core wire made of copper or a copper alloy containing tin, such core wire being further plated with gold so as to improve a breaking strength of the bonding wire. Further, in the patent document 2, although there is not disclosed a specific example of how a copper wire is coated with gold, there is disclosed the copper bonding wire with a noble metal such as gold, silver or the like coated on the copper wire. According to the patent document 2, there can be achieved a further improved corrosion resistance property with the copper wire thus coated. The patent document 3 discloses the bonding wire composed of the aluminum (Al) wire or the copper wire, such aluminum wire or copper wire being further plated with a noble metal such as gold, silver or the like. The copper bonding wire according to the patent document 3 can provide a solution to corrosion and to thermal oxidation due to the plating, and allows a reliable bondability to be achieved with respect to a lead frame as is the case in the gold bonding wire. The patent document 4 discloses the copper bonding wire with a noble metal or anticorrosion metal coated on the surface of the high-purity copper extra fine wire, such noble metal, however, being not limited to gold. This composition allows surface oxidation of the corresponding copper bonding wire to be controlled (particularly, the presence of surface oxidation after being left for 10 days in the atmosphere). Further, the diameter of the aforementioned copper extra fine wire is 15 to 80 μm, and the coating layer formed in this case has the average layer thickness of 10 nm to 1 μm (the diameter of the wire is 25 μm, and the average layer thickness of the coating layer is actually 0.1 μm in an example). A patent document 8 discloses coating of an outer periphery of a copper core wire with gold, such coating improving a bondability with respect to an electrode made of aluminum. A patent document 9 discloses a compound conductor composed of a core material that does not plastically deform, and an outer peripheral material softer than the core material and capable of undergoing plastic deformation. As an example, gold and a copper alloy are employed as the core material and the outer peripheral material, respectively, thus improving a connection strength between the conductive wire and a circuit. A patent document 10 discloses coating of an outside of a copper alloy with gold or a gold alloy, such coating preventing bonding wires from coming in contact with one another at the time of resin-sealing a semiconductor device. A patent document 11 discloses how pure gold is used to plate a surface of a wire rod made of an oxygen-free copper wire, and a bonding wire with a high signal conductivity and superior in high-frequency transmission accordingly. A patent document 12 discloses a bonding wire comprising: a core material mainly composed of copper; a dissimilar metal layer formed on the core material and made of a metal other than copper; and a coating layer formed on the dissimilar metal layer and made of an oxidation-resistant metal having a melting point higher than that of copper. The bonding wire according to the patent document 12 allows a spherical ball to be formed stably, and exhibits a superior adhesion between the coating layer and the core material.
However, with regard to the aforementioned copper bonding wires with silver or gold coated on the surfaces thereof, although there can be controlled surface oxidation of copper (particularly, progress of oxidation during storage), it is highly likely that the balls formed on the front ends of the bonding wires are deformed and fail to become spheres when performing bonding, thus preventing the corresponding copper bonding wires from being put to practical use. This is because, when coated with silver, such silver with a low melting point (melting point 961° C.) is preferentially melted as the front end of the bonding wire is being heated and melted through the arc heat input, whereas copper with a high melting point (melting point 1083° C.) is only partially melted at that time. Further, when coated with gold, copper with a large specific heat (380 J/kg·K) is not easily melted as the front end of the bonding wire is being heated and melted through the arc heat input, whereas gold with a small specific heat (128 J/kg·K) can be melted even with a small heat input at that time, thus bringing about a result in which gold in the multilayered structural body of copper and gold is preferentially melted. Further, as described in the patent document 5, there can be often formed a ball with a favorable shape if performing bonding under a reductive atmosphere (10% H2—N2). However, oxidation at the time of melting cannot be controlled if performing bonding under an atmosphere not containing hydrogen, thus making it difficult to perform bonding and impossible to form a ball with a favorable shape.
Meanwhile, instead of employing silver or gold, palladium (Pd) can also be conceivably used to coat a surface of a copper wire. In fact, the patent documents 2 through 4 also disclose palladium as a noble meal other than gold and silver used in the coating layers. Here, although the aforementioned documents do not specifically show a superiority of palladium, the truth is that palladium has a melting point (melting point 1554° C.) higher than that of silver and a specific heat (244 J/kg·K) larger than that of gold. Thus, when coated with palladium, there can be conceivably avoided a situation in which a spherical ball fails to be formed due to the fact that the coating layer is melted before the copper wire is melted to form the ball, as is the case in silver and gold. Namely, it is assumed that there can be simultaneously ensured both an oxidation resistivity of copper and the sphericity of the ball by coating the surface of the copper wire with palladium. The patent document 6 discloses how a diffusion layer is provided between a core wire and a coating layer (outer peripheral portion) of a double-layered bonding wire so as to improve an adhesion or the like of the corresponding coating layer. The patent document 6 discloses an example in which copper is employed as the core wire, and palladium is employed as the coating layer. With regard to such a kind of copper bonding wire coated with palladium, oxidation of copper is controlled, thereby not only allowing there to be achieved a superior long-term storageability and wedge bonding property, but also significantly reducing the possibility of oxidation of a ball being formed on a front end of the corresponding bonding wire. Accordingly, there can be formed a spherical ball by simply surrounding the ball with a nitrogen atmosphere prepared using a pure nitrogen gas, without using a hazardous gas such as hydrogen.